1. Field of the Invention
This invention relates to the field of solid-state power lasers pumped by optical sources, such as laser diodes.
2. Description of Related Art
High energy laser beams are useful as an industrial lasers, laser weapons or as optical pumps of X-ray lasers.
Generally, the laser effect involves a stimulated emission corresponding to the release of photons. Photons are emitted when electrons fall to a lower energy state. Stimulated emission provides induced radiation which has the same phase as the inducing radiation. Stimulated emission radiation has the same propagation direction and polarization so that stimulated emission is identical to the inducing radiation. Therefore stimulated emission provides phase coherent light amplification.
Radiation transmitted into any given medium has a much greater probability of being absorbed than of causing stimulated emission. To generate stimulated emission, a population inversion must be induced in the medium. A population inversion is defined to be a state in which a selected set of equivalent energy levels are more populated than another set of equivalent but lower energy levels. A population inversion is caused by an outside excitation which is usually called pumping.
The laser amplifier is composed of an active material that can be stimulated by a pumping source to generate stimulated emission. If a laser amplifier is placed in a resonant cavity, the laser amplifier becomes a generator of intense radiation.
Solid-state lasers generally consist of glasses or crystals which are doped with ions. The positions of the ion energy levels are allow population inversion to occur. Optical pumping is possible in solid state lasers, in contrast with gas lasers which are pumped by electric discharge because of the low optical absorption coefficient gas lasers.
The active media of solid-state lasers may be ruby, which is transparent alumina crystal incorporating trivalent chromium ions.
The active media may also be neodymium glass doped with Nd.sup.3+ ions or else yttrium-aluminum garnet (called YAG) also doped with Nd.sup.3+ ions The neodymium and YAG, Nd+3 media have a low population inversion threshold. These media have provide stimulated emission due to an electronic energy level system with four energy levels. Radiative emission is promoted in these 4 level systems through nonradiative emission to a lower energy from the lower level associated with the radiation transition. Nonradiative emission of electrons from radiative ground state to a lower state allows population inversion to be easily obtained.
The main pumping sources for the aforementioned solid state lasers have been flash lamps. Flash lamps emit in many directions and in a wide spectral band. The effective pumping energy provided by flash lamps represents only a small part of the energy dissipated by the flash.
The pumping efficiency is improved when laser diodes which have spectral emission centered on the absorption band of the doping ion are used to pump the media.
However, for high energy (several kilowatt) solid-state lasers, it is essential to generate a very high pumping power and intensity (several tens of kw/cm.sup.2).
Very high pumping power causes major heating problems. When laser diodes are used for pumping very high power lasers, excessive heating of the laser diodes and of the active solid material (laser media) occurs.
Conversion efficiency of electric energy into optical energy in laser diodes barely exceeds 40%. Conversion efficiency of optical pumping energy into radiative optical energy in the active material is barely greater than 30%. A considerable portion of the energy which is not converted into optical energy is absorbed and converted into heat in the active media.
Heat buildup limits the performance of such high power lasers. Therefore, providing a laser geometry which can conduct heat away from the active medium and choosing an active medium with high heat conductivity is very important. The material used for the active media must also exhibit low values of the thermal expansion coefficient and high thermal conductivity.
Low thermal expansion coefficients avoids stresses between the surface of and the center of the laser medium during when the laser medium is heated. Stresses result in the creation of irreversible defects (breaking of the material). Stresses also induces, through photoelastic interactions, inhomogeneous index of refraction changes in the medium.
A high thermal conductivity effectively dissipates heat from the center to the surface of the active laser medium and results in a lower temperature gradient and less stress in the active medium.
One solution to the heating problem has been to displace the rod constituting the solid active medium relative to a flash lamp thus promoting the evacuation of heat. However, any displacement of the rod may cause operating instabilities.